Note: Descriptions are shown in the official language in which they were submitted.
WO 2010/113084 PCT/IB2010/051308
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ORGANIC LIGHT EMITTING DIODE WITH BUCKLING RESISTING PROPERTIES FOR
LIGHT-INDUCED PATTERNING THEREOF
FIELD OF THE INVENTION
The invention relates to an OLED comprising a stack of layers, the stack
comprising a light emitting layer arranged between a cathode layer and an
anode layer, the
stack being arranged on a substrate.
The invention further relates to a patterned OLED and to a light source.
BACKGROUND OF THE INVENTION
An organic light emitting diode, also referred to as OLED, typically comprises
a cathode, an anode and a light emitting layer. These layers can be stacked on
a substrate.
The OLED may also comprise conductive layers. The light emitting layer may be
manufactured of organic material that can conduct an electric current. When a
voltage is
applied across the cathode and anode, electrons travel from the cathode
towards the anode.
Furthermore, holes are created in the conductive layer at the anode side. When
electrons and
holes recombine, photons are emitted from the organic LED device. Organic LED
devices
are in many ways considered as the future in various lighting applications.
Patent application `Device, method and system for lighting,' with attorney
docket PH009044, incorporated herein by reference, describes an organic LED
device. The
organic LED device displays, when in use, a predetermined pattern on its light
emitting parts.
The organic LED device comprises an anode, a cathode, and an organic light
emitting layer.
The organic light emitting layer is configured to emit light. Part of the
organic light emitting
layer stack has been irradiated by light with a wavelength in the absorption
band of the
organic light emitting layer. The light intensity of the irradiating light is
below an ablation
threshold of the cathode layer, the anode layer and the organic light emitting
layer. As a
result of the irradiation treatment that part of the light emitting layer
stack has reduced light
emitting properties.
By selecting which parts of the light emitting layer to treat, and for how
long,
an image may be imprinted in the OLED. Patterned OLEDs may, for instance, be
used to
create ambient lighting. Full 2-dimensional grayscale pictures can be made in
a single
organic LED device, while maintaining all intrinsic advantages of organic LED
devices, for
instance, being appealing, being a diffuse area light source and so on.
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SUMMARY OF THE INVENTION
During the patterning typically a condensed light beam, such as a laser, is
used. The laser has an intensity which is relatively high such that at a
location in the light
emitting layer stack that is irradiated with light, the OLED will heat up. To
avoid
deformation of the OLED, the irradiation-induced temperature in the OLED
should stay
below a deformation threshold. Patterning an OLED requires careful calibration
and control
of the laser intensity, as well as the scanning speed in order to get high
contrast patterning
without causing unwanted deformation of the metal electrode in the device,
i.e., buckling.
Especially the cathode layer is sensitive to buckling.
Moreover, in order to speed up the production of light-induced patterned
OLEDs it is desired to increase the intensity of the patterning light. When
the OLED is
heated, parts of the OLED may deform and eventually buckle.
To better address this concern, in a first aspect of the invention an OLED for
light-induced patterning is presented. The OLED comprises a stack of layers.
The stack
comprising at least a light emitting layer arranged between a cathode layer
and an anode
layer. The stack is arranged on a substrate. The OLED further comprises a
buckling-reducing
layer, not-being the substrate. The buckling-reducing layer is connected to
the cathode at a
side of the cathode layer facing away from the light emitting layer. The
buckling-reducing
layer is configured for improving a resistance to buckling resulting from
local heating of the
cathode.
The OLED for light-induced patterning according to the invention has the
advantage that it can be patterned with light in a more cost-effective manner.
In the known system, when light is applied at some point of the light emitting
layer stack to reduce the light emitting properties at that point, then the
light will also heat the
cathode layer. If the intensity of the light is high enough, then at some
point the cathode layer
will reach a temperature, at which it buckles.
However in the OLED according the invention, the cathode is connected to a
buckling-reducing layer, which increases the cathode's resistance to bucking.
Even if a
cathode layer were used of the same material and thickness as in the known
system, then the
cathode layer would be able to withstand buckling better.
Since the buckling-reducing layer is applied to a side of the cathode which
faces away from the light-emitting layer, the light-emitting properties of the
light-emitting
layer are not impaired.
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Because of the buckling-reducing layer, the intensity of light which is used
to
induce a pattern in the OLED may be increased. As a result less time is
required at one point
of the OLED to reduce the light emitting property of the light emitting layer
stack. The
scanning speed, with which the laser scans over the surface of the OLED during
the
patterning, can be therefore be increased. That is, if less time is required
for any particular
point of the OLED to achieve the desired altering of the light emissive
properties then also
less time is required to apply the whole pattern. Accordingly, the time for
pattering the
OLED is reduced. A shorter patterning phase during manufacture of a patterned
OLED
implies a corresponding shortening of the manufacture time of the patterned
OLED.
It is also possible to divide the scanning phase into multiple scanning
passes.
The reduction in the light emissive properties may then proceed in several
distinct steps. This
has the advantage that heat which is build up during a first pass can
dissipate before the
second pass starts. In this way buckling is avoided. When multiple scanning
passes are used
to pattern the OLED according to the invention, then fewer passes may suffice.
Since the
cathode layer has a higher resistance to buckling, the intensity of the laser
used in any one of
the multiple passes can be higher and fewer passes are needed. Fewer scanning
passes reduce
the time the patterning phase takes.
The manufacture time needed to fabricate a patterned OLED makes an
important contribution to the cost price of a patterned OLED. To reduce cost-
price, increased
patterning speeds are therefore of advantage.
A further advantage of using higher intensity light during the patterning is
that
the contrast in the pattern which may be achieved in a single pass is
increased. A higher
intensity light source can achieve a stronger reduction of the light emissive
properties of the
light emissive layer. Accordingly, a larger difference between darkened parts
of the OLED
and parts which are left untreated can be accomplished.
An OLED according to the invention may be used with various light-induced
patterning methods. As a first example, the light emitting layer may comprise
oligomers
and/or polymers and be patterned with a method which influences those
materials. As a
further example, the stack and/or the light emitting layer may comprise a
working layer, such
as a current support layer. In that case, the light induced patterning can
affect its current
supporting properties, so as to effect a reduced potential for current flowing
through the light-
emitting layer. If the potential for current flowing through the light-
emitting layer is reduced,
then the light emitting properties are correspondingly reduced. It is noted,
that in both
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examples, the light used will at least to some extent heat the cathode layer.
Accordingly, in
both examples a buckling-reducing layer will benefit the production process.
The higher resistance to buckling of the cathode layer may materialize in at
least two different ways.
First of all, an OLED may have a higher resistance to buckling of the cathode
layer by delaying the onset of the buckling. That is, by an increased buckling
threshold of the
cathode layer. The buckling threshold defining an amount of heat energy above
which
buckling of the cathode layer occurs, if said amount is applied to the cathode
layer during the
light-induced patterning.
By increasing the buckling threshold the intensity of the light can be
increased,
while avoiding buckling all together. Especially for cathode layers made from
fragile
materials, e.g., transparent cathode layers, staying under the buckling
threshold is preferred.
Compared to an OLED without a buckling reducing layer, the buckling would
start after
more heat-energy has been applied, since the buckling layer, e.g., withstands
the buckling
due to its stiffness, or because it assists in handling the incoming heat
energy. Higher light
intensities can be used without buckling.
A second way in which an OLED may have a higher resistance to buckling is
by mitigating the severity of the buckling after its onset. When buckling of
the cathode layer
has started but the application of heat continues, the buckling becomes
increasingly severe.
The severity shows, e.g., through higher and/or sharper folds of the material.
However, for
some applications, a certain amount of buckling can be tolerated as long as
the buckling stays
under predetermined limits. Particularly, the buckling should not progress to
the point where
the cathode ablates. A buckling reducing layer can slow the rate at which the
buckling of the
cathode layer progresses. Moreover, it reduces the visibility of the buckling.
In a preferred embodiment, the connection between the buckling-reducing
layer and the cathode layer comprises a mechanical connection for increasing a
mechanical
stiffness level of the cathode layer. A stiffer layer will be able to
withstand higher light
intensities, i.e., higher temperatures before buckling occurs.
Heating a part of the cathode layer during light-induced patterning causes
stress in the material. When this stress is sufficiently high, buckling
results. Having a
mechanical connection between the cathode layer and the buckling resisting
layer allows the
cathode to withstand a larger amount of stress. It is preferred to arrange the
buckling-
reducing layer to have a higher mechanical stiffness level than the cathode
layer, for
example, by selecting a suitable material or deposition method for the
buckling-reducing
WO 2010/113084 PCT/IB2010/051308
layer. Having a buckling-reducing layer with a higher stiffness than the
cathode layer, allows
the buckling-reducing layer to be thinner. Preferably, the mechanical
stiffness of the
buckling-reducing layer is not lower than the mechanical stiffness level of
the cathode layer.
A thinner buckling-reducing layer can be applied, e.g., deposited, quicker,
which reduces
5 manufacture time of the OLED. The invention may be used in a cost effective
production of
OLEDS, which reduced manufacture times and reduced patterning times. Moreover,
if the
buckling reducing layer can be thinner than less material is required for the
buckling reducing
layer.
Preferably, the stiffness of the buckling-reducing layer extends in a
direction
parallel to the cathode layer for reducing buckling of the cathode layer.
Increasing the stiffness in a direction parallel to the cathode layer is
effective
to reduce buckling of the cathode layer. If the material resists movement in
this direction,
then the freedom of the cathode layer to wrinkle is correspondingly reduced.
In a preferred embodiment, the connection between the buckling-reducing
layer and the cathode layer comprises a thermal connection for transporting
heat from the
cathode layer to at least part of the buckling-reducing layer.
The rate at which the buckling progresses after it has begun can be diminished
by transporting away some of the heat caused in the cathode by the impinging
light during
the patterning. In this way, although heat continues to be supplied to the
cathode, the severity
of the buckling is limited.
In a preferred embodiment, the buckling reducing layer and the thermal
connection to the cathode layer are configured to increase the buckling
threshold by
transporting heat to limit the local heating of the cathode layer during the
light-induced
patterning of the OLED. By transporting heat away from the cathode layer, the
build-up of
heat therein is prevented. Compared with the OLED without the buckling
resisting layer, the
onset of buckling will occur later, that is, after light has been applied for
longer and/or after
light of a higher intensity has been applied. Accordingly, light of a higher
intensity may be
used or light of the same intensity may be used for a longer time.
Better thermal conduction results in a lower temperature while the same light
intensity is used for patterning. This allows a higher thermal load, i.e.,
amount of heat-
energy, and consequently a higher light intensity.
In an embodiment according to the invention, the buckling-reducing layer and
the thermal connection between the buckling-reducing layer and the cathode
layer are
configured to transport heat away from the cathode layer to a further heat
sink. In this way,
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the capacity of the system formed by the cathode layer and the buckling
reducing layer to
handle inflow of heat is further increased. The heat-sink may be arranged in
the OLED, but
may also be arranged external to the OLED, and connected via a further thermal
coupling.
For example, a temporary heat-sink may be coupled to the buckling-reducing
layer during the
application of a pattern in the OLED using condensed light.
In a preferred embodiment, the buckling-reducing layer comprises a heat
capacity for absorbing heat to limit the local heating of the cathode layer
during the light-
induced patterning of the OLED to increase the buckling threshold. Having a
relatively high
heat capacity enables the buckling-reducing to absorb a considerable amount of
energy while
the increase in temperature remains limited. During light-induced patterning,
the heat
capacity absorbs part of the heat that is applied to the cathode layer. In
this way the buckling
threshold is increased.
The buckling reducing layer in an OLED according to the invention may
comprise materials whose material properties in semi-conductor and/or thin-
film fabrication
environments are well-understood. Such materials include various metals,
including
aluminum alloys, molybdenum, copper, and tungsten. Moreover, silicon is also
well suited.
Glass -like and ceramic materials are also possible, in particular solgel
materials which can
be applied in liquid form before curing. Preferably, the buckling reducing
layer comprises at
least one material out of the following list of materials: Aluminum Nitride,
Silicium Nitride,
SiNx:H, Aluminum Oxide, Aluminum oxynitride, silicon oxide or silicon
oxynitride. The
methods and equipment for applying coatings of these materials are commonly
available.
In a preferred embodiment according to the invention, the cathode layer and
the buckling reducing layer are at least partially transparent to visible
light. When the cathode
layer and the buckling reducing layer are transparent to visible light, the
OLED can emit light
in the direction of the cathode, possibly in addition to emitting light in the
direction of the
anode. Moreover, such an OLED can be at least partially transparent to visible
light. In the
latter case, the stack of layers, the substrate and the buckling-reducing
layer are also at least
partially transparent to visible light.
The cathode layer in a transparent OLED is typically a thin silver layer,
e.g.,
10 nm of silver. Such materials are especially sensitive to buckling. Because
such materials
are thinner they have a lower capacity for absorbing heat-energy. Also thin
materials are
damaged more easily. By applying a buckling reducing layer which is also
transparent to
light, the buckling in this type of OLED can be significantly reduced.
Transparent buckling
reducing layers may be fabricated from known materials, for example, the
buckling-reducing
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layer may comprise at least one material out of the following list of
materials: solgel, spin-on
glass or epoxy, Aluminum Nitride, Silicium Nitride, SiNx:H, Aluminum Oxide,
Aluminum
oxynitride, silicon oxide or silicon oxynitride .
Transparent SiN and Transparent AlO, are preferably used in amorphous, non-
crystalline form. Through the deposition technique their composition and
structure can be
varied, and consequently, their absorption.
In addition to or instead of visible light, the cathode layer and the buckling
reducing layer may also be at least partially transparent to UV light and/or
infrared light.
A further aspect of the invention concerns a patterned OLED according to the
invention, wherein part of the light emitting layer has locally reduced light
emitting
properties constituting a pattern. Said patterned OLED comprises a stack of
layers, the stack
comprising a light emitting layer arranged between a cathode layer and an
anode layer, the
stack being arranged on a substrate. The patterned OLED further comprises a
buckling-
reducing layer, not-being the substrate or the cathode, the buckling-reducing
layer being
connected to the cathode at a side of the cathode layer facing away from the
light emitting
layer, and being configured for improving a resistance to buckling resulting
from local
heating of the cathode. At least part of the light emitting layer has reduced
light emitting
properties through the application of light.
An OLED for light-induced patterning which is patterned according to a
suitable light-induced patterning method can be manufactured faster due to the
higher light
intensity which may be used. That is, the patterning costs of such patterned
OLEDs are
lower.
In a further aspect of the invention, a light source comprises a patterned
OLED
according to the invention. For example, in an embodiment, a lamp comprises a
patterned
OLED according to the invention.
An organic light emitting diode (OLED) for light-induced patterning is
presented. The OLED comprises a buckling-reducing layer connected to a cathode
layer at a
side of the cathode layer facing away from a light emitting layer. The
buckling reducing layer
is configured for improving a resistance to buckling resulting from local
heating of the
cathode, which heat may be caused by patterning the OLED. The buckling
reducing layer
improves mechanical properties, e.g., stiffness, and/or thermal properties,
e.g. through
cooling, of the cathode.
It is to be noted that patent application, `Patterned OLED device, method of
generating a patterning, system for patterning and method of calibrating the
system', with
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attorney docket PHO12033, incorporated herein by reference, describes a
patterned light
emitting diode device. The patterned organic light emitting diode device
comprises organic
light emitting material arranged between an anode layer and a cathode layer
and further
comprises at least one current support layer for enabling a current flowing,
in operation,
through the light emitting material to cause the light emitting material to
emit light. Part of
the current support layer is patterned by locally altering a current support
characteristic, while
not substantially altering the organic light emitting material, the anode
layer, and the cathode
layer. The current support characteristic locally determines the current
flowing through the
organic light emitting material in operation. By altering the current support
characteristic, a
pattern may be created in the organic light emitting diode device which is
substantially not
visible in an off-state of the organic light emitting diode device, and which
is clearly visible
as light intensity variations in an on-state of the organic light emitting
diode device.
Modifying current support layers is particularly effective for oligomer-based
OLEDs. For polymer based OLEDs, it is preferred to modify the light emitting
material itself
through light irradiation. Such devices may not have a current support layer,
and it may be
slightly visible in an off-state of the device that the OLED is patterned.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in further detail by way of example and with
reference to the accompanying drawings, wherein:
Figure 1 a is a schematic cross-sectional view of an organic LED device
according to the invention,
Figure lb is a schematic cross-sectional view of the light emitting layer of
an
organic LED device according to the invention,
Figure 2 is a schematic cross-sectional view of a further organic LED device
according to the invention.
Throughout the Figures, similar or corresponding features are indicated by
same reference numerals.
List of Reference Numerals:
100 an OLED
110 a substrate
120 an anode
130 a light emitting layer
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132 a conductive layer
134 an emissive layer
140 a cathode
150 a buckling-reducing layer
160 a light emission direction
165 pattern inducing light
200 an OLED
260 a lighting direction
265 a pattern inducing direction
DETAILED EMBODIMENTS
While this invention is susceptible of embodiment in many different forms,
there is shown in the drawings and will herein be described in detail one or
more specific
embodiments, with the understanding that the present disclosure is to be
considered as
exemplary of the principles of the invention and not intended to limit the
invention to the
specific embodiments shown and described.
Figure 1 a shows a cross-sectional view of an organic LED device 100
according to an embodiment of the present invention. OLED 100 comprises a
substrate 110
on which are applied, in order, an anode 120, a light emitting layer 130, a
cathode 140 and a
buckling-reducing layer 150. Anode 120 may for instance comprise Indium tin-
oxide (ITO),
fluoridated Zinc-oxide, PEDOT, or any other suitable anode material. A voltage
can be
applied over cathode 140 and anode 120, resulting in a current flow through
light emitting
layer 130. Figure lb shows light emitting layer 130 in more detail, it
comprises a conductive
layer 132 and an emissive layer 134, wherein the conductive layer 132 is
towards the side of
the anode 120 and the emissive layer 134 towards the cathode 140. According to
the art of
OLEDs, intermediate layers may be present in the OLED. For example, current
support
layers may be present between anode 120 and cathode 140.
Conductive layer 132 and emissive layer 134 may be manufactured of an
organic material such as a polymer or an oligomer. Light emitting layer 130
may comprise
materials with low molecular weight, so-called small-molecule (SM) OLED. The
deposition
of SM-OLEDs is typically based on vacuum thermal evaporation. Light emitting
layer 130
may also be polymer based (PLEDs), comprising long polymer organic chains,
which may be
deposited by spin-cast or ink-jet principles.
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In order for OLED 100 to function properly and for protection from moisture
and contamination from, for instance, dust and small particles, the OLED 100
can be
encapsulated with an encapsulating body (not shown) such as an encapsulating
lid. When a
voltage is applied, electrons and holes recombine in organic light emitting
layer 130 which
5 causes light to be emitted from the OLED 100. The light can, for instance,
be emitted via the
anode 120, in which case anode 120 is at least partially transparent to the
generated light.
Light emitted through anode 120 is shown in Figure 1 a as light emission
direction 160.
Cathode 140 may also be transparent. Substrate 110 may also be transparent.
For example, Substrate 110 may be made of glass.
10 OLED 100 may be patterned by irradiating with pattern inducing light 165. A
light beam 165 irradiates OLED 100 causing the light emitting properties of
light emitting
layer 130 to be altered in the irradiated areas. Light beam 165 may, e.g.,
pass through
substrate 110 and anode 120 to affect the light emitting layer 130. Pattern
inducing light 165
may have a wavelength in the absorption band of light emitting layer 130, in
one embodiment
avoiding wavelengths below 400 nm. The photo-induced process in light emitting
layer 130
causes a reduction of the original light emission in the irradiated areas of
light emitting layer
130, allowing a pattern to be visible when OLED 100 is switched to its on-
state. In Figure la,
the pattern inducing light 165 reaches the light-emitting layer 130, through
the substrate 110
and anode 120, which are for that purpose at least partially transparent to
the patterning light
165. Alternatively, the light-emitting layer 130 may be reached through the
buckling
reducing layer 150, and the cathode 140. In the latter situation, the buckling
reducing layer
150 and the cathode 140 are at least partially transparent.
In one embodiment, pattern inducing light 165 is laser light. OLED 100 can,
for instance, be a known super-yellow device of bottom emission type, on a 0.5
mm soda-
lime glass substrate, on which a buckling reducing layer is deposited. Pattern
inducing light
165 may be generated by a frequency doubled Nd:YAG laser ( 532 nm wavelength).
In one embodiment, OLED 100 comprises a blue-emitting polymer. Pattern
inducing light 165 may have a wavelength of 405 nm. In this case, a low-price
solid state
diode laser as used in Blue-ray disc products may be used.
During light-induced patterning, condensed light impinges on the light
emitting layer for altering its light emissive properties. At least part of
that light also reaches
the cathode layer and impinges upon it, e.g., because some part of the light
transmits through
the light emissive layer. Due to partial absorption of this impinging light
the cathode is
heated.
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Buckling-reducing layer 150 is connected to cathode 140 to mitigate the
deforming effects due to local heating. The buckling threshold defines an
amount of supplied
energy above which buckling of the cathode layer occurs, if said amount is
applied to the
cathode layer during the light-induced patterning, e.g., during some pre-
determined time
period or at a pre-determined scanning speed of the pattern inducing light.
The buckling
threshold may also be expressed as a temperature increase of the cathode
layer, above which
buckling occurs. Buckling-reducing layer 150 can delay the onset of buckling
by increasing
the buckling threshold.
Moreover, even if buckling occurs then buckling-reducing layer 150 assists in
controlling it, i.e., reducing its severity. Preferably, the thermal and/or
mechanical connection
between buckling-reducing layer 150 and cathode 140 is relatively strong and
has a relatively
high adhesion. Buckling-reducing layer 150 can help resist deformation by
increasing the
stiffness of the cathode 140 and/or transporting at least part of the heat
applied to cathode 140
away from it.
For example, the connection between buckling-reducing layer 150 and cathode
140 may be chosen such that some of the forces which are caused in cathode 140
by the heat
are at least in part resisted by virtue of being connected to buckling-
reducing layer 150. In
other words, buckling-reducing layer 150 may act as a kind of skeleton for
cathode 140. The
stiffness of buckling-reducing layer 150 may be expressed in terms of its
Young's modulus
E. An improvement of the buckling resistance of cathode 140 has already been
observed
from an E value of the buckling reducing layer of 50 GPa. However, Young's
modulus of
buckling-reducing layer 150 is preferably greater than 100 GPa and more
preferably greater
than 250 GPa. Choosing materials with a high mechanical stiffness level for
the buckling-
reducing layer, in particular, higher than the mechanical stiffness level of
the cathode layer, is
an efficient way to increase the stiffness of cathode 140, especially when
combined with a
strong mechanical connection.
It is advantageous if buckling-reducing layer 150 itself does not deform
strongly in response to heat. The thermal expansion coefficient of buckling-
reducing layer
150 is therefore preferably small, e.g., smaller than 30 ppm/K (=10-6 / K),
and preferably
smaller than 10 ppm/K. If buckling-reducing layer 150 has a relatively low
thermal
expansion coefficient, e.g., lower than the thermal expansion coefficient of
the cathode layer,
then deformation in cathode 140 is resisted as well, especially when the
connection
comprises a mechanical connection.
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As a further example, buckling-reducing layer 150 can also help resist
deformation by transporting at least part of the thermal energy applied to
cathode 140 away
from it. The connection between cathode 140 and buckling-reducing layer 150
may comprise
a thermal connection for transporting heat from cathode 140 to at least part
of buckling-
reducing layer 150. As the heat is transported away, the buckling onset will
be delayed.
Moreover, after the onset of buckling, the buckling will proceed slower, since
some of the
heat is transported away. Preferably, the buckling-reducing layer has a heat
capacity so that
some of the heat which is transferred from the cathode layer 140 to the
buckling reducing
layer 150 may be absorbed by the buckling reducing layer 150, during the light-
induced
patterning of the OLED. This further increases the buckling threshold.
Preferably, the layer's
thermal capacity is greater than 2 J/cm3/K, and the layer has a high thermal
conductivity. A
relatively high thermal conductivity allows the heat energy which is absorbed
locally to be
transferred to other parts of the buckling reducing layer which are currently
irradiated by the
patterning light. In this way, the thermal conductivity assists in spreading
the heat energy
over a larger area of the buckling reducing layer. As a result, the overall
temperature increase
will be reduced and thus the capacity of the buckling reducing layer for
cooling the cathode
layer is increased. Moreover, if the heat is spread over a larger area, then
the buckling
reducing layer itself can also dissipate its heat-energy more easily.
A further heat sink (not shown) may be connected to cathode 140, via
buckling-reducing layer 150.
It has been observed that the above mentioned effects markedly increase with
the layer thickness of buckling-reducing layer 150. The layer thickness of
buckling-reducing
layer 150 is preferably greater than 20 nm, or greater than 50 nm or greater
than 100 nm.
Although it is preferred that buckling-reducing layer 150 is a separate layer
from cathode
140, it has been observed that an increase in buckling resistance can be
achieved by
increasing the thickness of cathode 140 itself, without using a separate
buckling-reducing
layer. For example, one embodiment of such OLED is an OLED comprising a stack
of layers,
the stack comprising a light emitting layer arranged between a cathode layer
and an anode
layer, the stack being arranged on a substrate, wherein part of the light
emitting layer has
locally reduced light emitting properties constituting a pattern, which
pattern is preferably
light, e.g., laser, induced, and wherein the cathode layer has a thickness for
improving a
resistance to buckling resulting from local heating of the cathode. The
cathode preferably
comprises aluminum, and may even consist of an aluminum alloy. A thicker
layer, e.g. metal
layer, has at least two advantages: enhanced cooling of the cathode due to
increased heat
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sinking capacity, and increased stiffness of the cathode. Both aspects help
preventing the
occurrence and extent of buckling during laser irradiation for patterning the
OLED. In this
way, the cathode has a higher resistance to buckling resulting from local
heating of the
cathode. Buckling of thicker materials produces less visible wrinkles in the
material.
Therefore, apart from making the cathode more robust against buckling, the
thicker layer,
makes buckling also less visible if it occurs. It is also shown that higher
contrasts in the
pattern can be achieved. Moreover, higher patterning speeds and higher light
power can be
used, which decreases production time. Preferably cathode 140 has a thickness
of at least 100
nm, or greater than 150nm, or greater than 200nm.
It has been observed that in this range the maximum light output of a
patterning laser without buckling increases approximately proportionally with
the thickness
of cathode 140 and/or buckling-reducing layer 150.
Example materials for buckling-reducing layer 150 include various metals,
including aluminum alloys, molybdenum, copper, and tungsten. These have a
relatively large
Young's Modulus and relatively small thermal expansion. Alternatively, silicon
is suited as
well. Silicon has similar properties as the mentioned metals, moreover it has
a relatively low
expansion. Glass, glass-like and ceramic materials are also possible, in
particular solgel
materials which can be applied to cathode 140 in liquid form before curing.
Preferred materials further include dielectrics, such as A1Nx, SiNx, SiN:H,
AlOx, A1ONx, etc. These materials have a relatively very large Young's Modulus
and
relatively small thermal expansion. Moreover, they can be readily deposited at
high rates and
at low cost in a normal production line as compared to the metal electrode
deposition. Using
these materials for buckling-reducing layer 150 is therefore advantageous for
fabrication, as
they lower the time needed for apply the buckling reducing layer.
Some example values of Young's elastic modulus (GPa) for various materials:
Al 69, glass 65-90, Cu 120, W 400, SiNx - 300, AlOx - 300; and of the thermal
expansion
(10-6 /K): Al 23, glass 3-8.5, Si 3, Mo 4.8, AlOx 6, SiN 2.5;
For transporting thermal energy, buckling-reducing layers comprising metal
are preferable, for example, using copper, aluminum and alloys comprising
them. Also
suitable are molybdenum and tungsten, which have advantageously a relatively
low thermal
expansion coefficient and high E modulus. Furthermore, silicon even in
amorphous form is
advantageous. Apart from the transparency, the glass-like and dielectric
materials are
particularly suitable for their high E modulus. AN is suitable for its high
conductivity.
WO 2010/113084 PCT/IB2010/051308
14
The stack of the anode layer 120, light emitting layer 130 and cathode layer
140 can be placed on substrate 110 either with the cathode layer 140 towards
substrate 110 or
with the anode layer 120 towards substrate 110. Shown in Figure 2, is OLED
200, which has
an alternative placement of the layers. Figure 2 shows a substrate 110 on
which is arranged,
in order, the buckling-reducing layer 150, the cathode 140, the light emitting
layer 130, and
the anode 120.
The arrangement in Figure 2 is suitable for top-emission. In Figure 2, light
is
emitted in a direction 260 and transmitted through anode 120, which is at
least partially
transparent to the emitted light. Applying the pattern may be done by a
condensed light beam
in a pattern inducing direction 265, that is, not through the substrate. In
case substrate 110 is
transparent to the used patterning light, it may also be done through
substrate 110.
When patterning is done through the substrate 110, a transparent cathode may
be used, such as thin silver layer. The silver layer has a thickness of
preferably less than
20nm. Transparent cathodes are particularly vulnerable to buckling during the
patterning.
Part of the light impinging on the cathode is absorbed by the cathode layer
causing local
temperature rise, and eventually buckling. Because of buckling-reducing layer
150, the
cathode 140 is protected from buckling along the same principles as explained
for Figure I a.
Preferably, when a, at least partially, transparent cathode is used, also a,
at least partially,
transparent buckling-reducing layer 150 is used. Suitable materials for a
transparent
buckling-reducing layer 150 include glass, transparent Silicon, Nitride,
transparent
Aluminum Oxide, etc (see above).
It is observed that in the arrangement of Figure 2 that buckling is more
problematic if the substrate 110 is of a material with a low Young's modulus,
such as plastic.
For the production of flexible devices, materials like PET or PEN can be used.
These have E
values in the range of 6 GPa, about an order of magnitude smaller than glass.
To prevent
moisture degradation of the OLED, barrier layers are typically applied on
these substrates.
One approach is to use a layer stack comprising, e.g., acrylic polymers in
combination with
thin inorganic layers. These polymer materials have even lower E values,
ranging from about
40 MPa up to 3 GPa.
It should be noted that the above-mentioned embodiments illustrate rather than
limit the invention, and that those skilled in the art will be able to design
many alternative
embodiments without departing from the scope of the appended claims. In the
claims, any
reference signs placed between parentheses shall not be construed as limiting
the claim. Use
of the verb "comprise" and its conjugations does not exclude the presence of
elements or
WO 2010/113084 PCT/IB2010/051308
steps other than those stated in a claim. The article "a" or "an" preceding an
element does not
exclude the presence of a plurality of such elements. The invention may be
implemented by
means of hardware comprising several distinct elements. In the device claim
enumerating
several means, several of these means may be embodied by one and the same item
of
5 hardware. The mere fact that certain measures are recited in mutually
different dependent
claims does not indicate that a combination of these measures cannot be used
to advantage.
